Appendix B - Stormwater Management Plan

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CONTENTS
SUMMARY .................................................................................................................................. 3
1.0     INTRODUCTION ................................................................................................................ 3
2.0     RESPONSE TO INFORMATION REQUEST .......................................................................... 3
3.0     SITE DESCRIPTION .......................................................................................................... 5
   3.1     Proposed Works ............................................................................................................. 5
4.0     WATER QUALITY MANAGEMENT PLAN.............................................................................. 6
   4.1     Proposed Stormwater Treatment System and Catchments ................................................... 6
      4.1.1     Bioretention Area ..................................................................................................... 6
5.0     FLOODING ASSESSMENT ................................................................................................. 8
   5.1     Existing Conditions.......................................................................................................... 8
   5.2     Proposed Drainage System .............................................................................................. 9
      5.2.1     Bioretention/Detention Capacity ................................................................................. 9
      5.2.2     Drainage Improvements/Non Worsening ..................................................................... 9
6.0     CONCLUSIONS ............................................................................................................... 12
7.0     APPENDIX – Council Master Stormwater Catchments ........................................................... 13




Eagle Rise Estate                                                                                                                pg. 2

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SUMMARY
The key points of this investigation are as follows:

     1.    Provide a Stormwater Management Report to Noosa Council Requirements.
     2.    Identify external catchment stormwater influence on mitigation measures.
     3.    Stormwater flows into and through the site including 29 Tait Street.
     4.    Capacity of downstream drainage system in Tait Street to cater for additional runoff.
     5.    Identify Detention basin and bio-retention dimensions.


1.0        INTRODUCTION
This integrated water management plan (IWMP) has been prepared for the new subdivision project at 20 Eagle Drive
Tewantin.

This Integrated Water Cycle Management Plan (IWMP) includes detail on the management of rainwater, and
stormwater flows.

2.0        RESPONSE TO RECONFIGURATION INFORMATION REQUEST
The enclosed report shows calculations to support proposed stormwater management solutions for quantity and
quality.

There will be some treatment of external stormwater as a result of proposed strategy. The external catchment
stormwater will be treated as follows;

          Stormwater Quality: Stormwater drainage pipes from subdivision will capture majority of minor flows (1.056
           Ha of 1.585 Ha) for treatment in bio-retention basin in no 14 Eagle drive. To avoid the need to directly treat
           lots that fall away from the road (proposed lots 1-5, 7-9) 0.529 Ha of external catchment will also be treated.

           To proportion external intake of remaining 0.529 Ha at Operational works detailed stormwater calculations
           of pit and pipe systems in the subdivision will be completed, water from upstream catchment and area
           around 14 Eagle drive and size an inlet/pipe system to suit. There may be additional pits outside no 14
           Eagle Drive to take additional flow as shown on plan 1702-02A.

          Stormwater Detention major road flows from external catchment will be directed to no. 14 Eagle Drive.
           Calculations have been completed to proportion flow as follows for Q100 (rational method):

                    1.   Subdivision Q100 overland water into detention basin to attenuate flow          0.175 m3/s
                    2.   Q5 subdivision piped flow into detention basin to attenuate flow                0.481 m3/s
                    3.   Upstream flow (already detained) less 0.05 m3/s roofwater                       0.124 m3/s
                    4.   Water from new sag pits at 14 Eagle Drive into detention basin attenuate flow   0.164 m3/s
                    5.   Overland flow down pathway at 14 Eagle Drive                                    1.101 m3/s
                    6.   Existing pipe flow from sag outside 14 Eagle Drive                              0.540 m3/s
                    7.   Overall flow of 4.514Ha external catchment to 14 Eagle Drive                    2.585 m3/s

           Note the sum of items 1-4 is 0.944 m3/s, which is the Q100 flow from 1.585 Ha of new subdivision to be
           attenuated in detention basin.

           Future upstream roof-water from behind lot 20 will be connected to the roof-water system through 29 Tait
           Street as noted in item 3. This pipe system in Tait Street was designed some time ago for this catchment.




Eagle Rise Estate                                                                                                pg. 3

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          The proposed easement through 29 Tait Street is not designed or required to take overland flow from the
           subdivision. It will only house a 300mm pipe for inter-lot drainage. As per following section 5.5.2 and below
           we explain and provide calculations that overland flows to properties fronting Tait Street in minor and major
           events will reduce due to new road and inter-lot stormwater pits in proposed lots 1-5 and 7-9.

           There are existing pipes in Tait Street that were designed to cater for inter-allotment flows from properties
           behind them. There is a 300mm concrete pipe parallel to Tait Street, with a maximum of two lots using the
           system at present for roofwater before they enter existing gully pit in Tait Street.
           Total usage of inter-lot stormwater:
                     2 existing lots
                     9 lots from 20 Eagle Drive subdivision
                     5 lots from 28 Elkhorn Drive
                     Total lots 16, at normal rates of 10l/s per lot this is 160l/s
                     300mm pipe capacity at 2.65% grade from survey of invert levels is 220l/s
                     Therefore existing pipe capacity of 220l/s is above required 160l/s maximum inflow

       As a comment on other existing drainage pipes in Tait Street, the overall catchment falls to Tait Street at
        present and overall flow to low point in road reserve opposite 25-27 Tait Street will be the same.

           The development will have detention for its own catchment to limit to pre-development levels. Given that the
           developer is already spending a significant amount of money on purchasing 14 Eagle Drive and improving
           many existing drainage issues it is unreasonable to consider further improvements downstream on Tait
           Street.

           Also we note that Master Drainage Catchment (see Appendix) would have been used to design the existing
           stormwater system in Tait Street, with catchment and developed flows as per developed catchment (see
           Plan 1702-01B). However Operational Works detail design will need to investigate detention basin outlet
           into existing stormwater system is acceptable, but extent of works would be limited to basin requirements.




Eagle Rise Estate                                                                                                pg. 4

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3.0        SITE DESCRIPTION
The site is located at 20 Eagle Drive Tewantin (Lot 6 RP149498), 35A Tait Street Tewantin (Lot 10 SP241386) and
14 Eagle Drive Tewantin (Lot 31 RP149497). The property at 29 Tait Street Tewantin (Lot 3 RP149498) is not part of
the development but has a piped inter-lot stormwater easement proposed, the existing owner has agreed to this.

The main area at 20 Eagle Drive Tewantin is grass and regrowth trees with an existing house, the other allotments
listed have houses on them at present. The surrounding area is developed urban areas except the adjacent lot at 28
Elkhorn Drive (Lot 70 M37807).

3.1        Proposed Works

The overall proposal is to construct 21 allotments and demolish the house at 14 Eagle Drive Tewantin (Lot 31
RP149497) to create a detention/bio-retention basin and overland flow path. The house at 35A Tait Street Tewantin
(Lot 10 SP241386) will remain unchanged and will therefore be left out of overall treatment/detention catchment. The
house at main allotment on 20 Eagle Drive Tewantin (Lot 6 RP149498) will be demolished to create three new lots.

As a result of these works water cycle management areas include:
     Stormwater drainage and treatment devices are required to drain the site, attenuate flows and treat
         stormwater runoff to acceptable levels to meet water quality objectives.

Plans 1702-01B shows the stormwater layout for the site, and plan 1702-02A proposed detention/bio-retention basin.




Eagle Rise Estate                                                                                           pg. 5

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4.0        WATER QUALITY MANAGEMENT PLAN
4.1        Proposed Stormwater Treatment System and Catchment

The developed site will be treated by the Bio-retention basin at 14 Eagle Drive.

Due to the existing topography, proposed lots 1-5 and 7-9 fall away from the road. Rather than install rain-gardens
and detention in these lots the proposal is to treat these areas in one location to make maintenance easier. This
involves treatment of adjacent existing catchment instead. This is considered reasonable to make the end results of
flow detention and treatment possible in the same catchment.

The developed site area for stormwater quality treatment is as per Table 3.1, and Plan 1702-01B:

TABLE 4.1            CATCHMENT AREAS FOR STORMWATER TREATMENT AND DETENTION (DEVELOPED)

                    Catchment                       Land Use         Original Area (ha)      Treatment Area (ha)

                      A+B                         Urban + Road              1.585                    1.585
20 Eagle Drive Tewantin plus proposed lot 7
 (part of 35A Tait Street Tewantin). Existing
  house on 0.06Ha to remain so this area is
           excluded from catchment

 14 Eagle Drive Tewantin (demolish house)             Urban                 0.845                      0

             TOTAL                                                                                 1.585Ha



Based on a percentage of site area, the requirement stormwater quality treatment area will be:

1.5% of 1.585Ha = 0.024Ha or 240sqm (area subject to detail design at Operational Works)

Discharge points will include the:
     Detention basin overland overflow into the adjacent park at 15 Tait Street (Lot 42 RP149497)
     Piped connection from detention basin to stormwater system at 15 Tait Street (Lot 42 RP149497)

The detention/bio-retention component of these treatment systems will remove the nutrients, heavy metals, and
retard flow.

Water Quality Management

The use of a bioretention basin is required for the site to treat pollutant loads generated from the proposed increase
in impervious area. As this is at a Reconfiguration stage the extent of information is based on estimation of required
bio-retention basin treatment area. Detailed calculations will be provided at Operational Works phase.

4.1.1 Bioretention Area
The Bioretention basin (see Plan 1702-02A, 1702-03A) has the following features:
      Filter Area = 240m2 (subject to detail design)
      Filter depth = 0.5m
      Transition layers depth = 0.1m



Eagle Rise Estate                                                                                             pg. 6

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          Detention depth = 0.2m
          Total filter + transition depth = 0.6m
          It is to be vegetated with nutrient effective removing plants.

Lining:

The surrounding soils are clay to silty clays with a saturated hydraulic conductivity of approximately 3.6mm/hr. The
sandy loam media that is proposed as the filter media has a hydraulic conductivity of 382mm/hr, therefore the
conductivity of the filter media is > 10 times the conductivity of the surrounding soils and an impervious liner is not
considered to be required.




Eagle Rise Estate                                                                                               pg. 7

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5.0       FLOODING ASSESSMENT

5.1       Existing Conditions

The summary below is taken from a previous investigation by Callaghan and Toth, with some updates to current
proposal:

5.1.1     Currently there is no major flood dedicated overland flow path for the runoff from the 3.323ha catchment
          (4.514Ha developed as per Council Master Catchments no 2153 & 2154 in the Appendix and developed
          catchment on plan 1702-01B) to the east of No.14 Eagle Drive (note: of the 4.514ha, our clients land
          falling to this point represents 1.056ha). The problem is also compounded by the fact that existing gully
          pits and pipe drainage at the sag at No.14 Eagle Drive only have the capacity to take a one year storm
          runoff. Therefore runoff overtops the kerb regularly and floods No.14 Eagle Drive and in large storms
          the runoff also floods through No.16 Eagle Drive.

          Both of those occurrences have been verified on site by the owner of No.16 and Rental Agent for No.14.

          The Owner of No.16 said that in large storms the runoff floods No.14 and has entered the house, it also
          backs up and floods down his (No.16) driveway and floods his shed in the south west corner.

          The real estate agent handling the rental of No.14 said that carpet has been replaced as a result of
          flooding, sand bags have been used in other events to stop water entering the house, and that a mound
          has had to be built in the front yard to divert flooding from the road away from the front door and down
          the side of the house.

          A further problem is that this house is also built only 2.7m away from the boundary adjacent where it
          floods through the land. In this location there is not enough room for a Council 3m easement or to lay
          pipes without impact on the house footings.

5.1.2     It can be seen at Eagle Drive (in sag area opposite No.14) that Council has recently replaced and
          lowered the grates to the gully pits in this location in an attempt to alleviate a problem.

5.1.3     The existing undersized stormwater pipe from the sag gully pits at No.14 does not go down the side of
          the house in the lowest point in the street, instead it goes down the north side of No.12.

          This pipe has no easement and the house is built right next to the pipe approximately 0.8m to 1.1m off
          the side boundary. These facts mean this pipe cannot be replaced by a larger pipe to reduce flooding.

5.1.4     A separate flooding problem exists for the properties 31, 29, 27 and 25 Tait Street. Our Clients 1.191ha
          of un-developed land slopes toward the rear of these properties and floods through their yards onto Tait
          Street (see plan 1702-01B).

      This problem was addressed by Council approximately 15 years ago by the construction of an extension of
      the underground drainage system along the frontages of these properties.

      Callaghan and Toth designed this system for Council. The system is in place even though no records exist.
      Its purpose was to allow residents to run a stormwater pipe from our Clients land boundary through to the
      piped extension so as to alleviate the flooding. Unfortunately from discussion with owners they were not
      aware of this option.



                                                                                                               pg. 8

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      Unfortunately there is no room between houses 31, 29, 27 and 25 Tait Street and their side boundaries to
      allow a 3m wide overland flow path and underground drainage. There is however enough for a 1.5m wide
      inter-lot drainage outlet which our Client has negotiated permission from the Owner to construct. This will
      allow properties in our Clients development on the low side of the new road to drain to Tait Street.

5.2       Proposed Drainage System

The proposed road through new estate at 20 Eagle Drive Tewantin (Lot 6 RP149498) has a catchment generally
in accordance with Council’s original master drainage plan (Council Master Catchments no 2153 & 2154 in the
Appendix). There is some un-avoidable diversion of flow as a result of road construction, hence the importance
of providing an overland flow path through the low point (sag) at 14 Eagle Drive. The overall catchment flow to
sag point in Tait Street road reserve (opposite 25-27 Tait Street) is however unchanged.

The benefits of the local community are significant for installing a bio-retention/detention basin at 14 Eagle Drive
and are listed below:
     Provide a Q100 overland flow path which the existing catchment outside the development needs.
     Provide 845m2 of additional park/drainage reserve which will extend from the existing park to Eagle
          Drive
     Removal of the regular flooding issues associated with No. 14 Eagle Drive that Council would eventually
          have to address.

5.2.1     Bioretention/Detention Basin Capacity

The Bioretention/Detention basin size at 14 Eagle Drive is only designed for reducing main catchment at 20
Eagle Drive Tewantin (Lot 6 RP149498), and proposed lot 7 on 35A Tait Street Tewantin (Lot 10 SP241386) with
an area of 1.585Ha (refer plan 1702-01B). The previous smaller detention basin on plan 1403-15A was designed
for a catchment of 1.04Ha (lots 1-5 and 7-9 not included). The detention basin has been made narrower and
longer to provide more space around it and treat the larger area with a 420cum volume, refer to plan 1702-02A
and sections on plan 1702-03A.

5.2.2     Drainage Improvements/Non Worsening

Point 5.1.1 – 5.1.3: Properties at 14 and 16 Eagle Drive.

The property at 14 Eagle Drive is to be demolished to provide an overland flow path from surrounding area.

This will reduce issues at 16 Eagle Drive because as noted the lack of flow path through sag point backs water
up in the road which eventually flows into 16 Eagle Drive.

It is also planned to install a footpath in front of 16 and 18 Eagle Drive to obtain a minimum 100mm verge height
above top of kerb. Works will also involve driveway revisions to maintain the 100mm verge height at 16 and 18
Eagle Drive. The verge at 12 Eagle Drive will have minor fill of approximately 100mm and turf to maintain 100mm
height of verge above top of kerb level. Refer to plan 1702-02A. These works will also enable the northern Q100
road flows from new subdivision and existing area of 1.92Ha (1.128cumecs) to get to sag at 14 Eagle drive with
flow a maximum of 50mm above top of kerb to QUDM flow limitations.

The proposed overland flow path and depths are shown on plan 1702-03A. The overland flow in Q100 is below
maximum dV of 0.6m/s², and while a lower dV would be preferred the path exits to open grassed parkland with
shallow slow moving sheet flow and no adjacent inlet system. The outlet location is therefore low risk to
requirements of QUDM.




                                                                                                                pg. 9

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Other adjacent properties at 21 and 23 Tait Street.

It is likely that 21 and 23 Tait Street have existing overland flow issues from upstream properties at 16 and
possibly 18 Eagle Drive. The improvements obtained by having a flow path through 14 Eagle Drive and raising
verge of Eagle drive to take road flows will also reduce overland flow at these properties.

In addition it is proposed to add a landscaped bund beside 21 Tait Street to ensure that overland flow through
park has 300mm freeboard to QUDM requirements. The property at 21 Tait Street has a floor level close to park
level and verbal advice indicates there is existing flooding from park flow path which heads in a north west
direction (refer to contours on plan 1702-02A and 1702-02A).

As previously noted there will be an increase of flow to 14 Eagle Drive due to subdivision road construction which
is un-avoidable and as per Council drainage catchments and natural extension of Eagle Drive. This also means
there will be increased flow through Park at 15 Tait Street, even though subdivisions upstream will limit flow to
pre-developed amounts by detention. To protect 21 Eagle drive from Park overland flow, a landscaped bund will
be added to protect the property and provide 300mm freeboard as per plan 1702-02A. Once flow enters Tait
Street the water continues to existing sag and west into Road reserve/channel, where adjacent houses are well
above road levels. The Tait Street stormwater system was designed to Council master stormwater plans so the
proposed flow to this location was previously planned for.

Point 5.1.4: Properties at 31, 29, 27 and 25 Tait Street.

29 Tait Street: Existing Catchment =            0.3Ha,
                                                180m length of grass/treed area at 3.4%, tc = 18 minutes,
                                                fi = 0, C10 = 0.70, C5 = 0.67, C100 = 0.84.

                                                Q5        = FCIA = 1/360 x 0.67 x 121 x 0.3 = 0.068cumecs

                                                Q100      = FCIA = 1/360 x 0.84 x 195 x 0.3 = 0.137cumecs


29 Tait Street: Developed Catchment =           0.06Ha,
                                                Urban lot area at 3.4%, tc = 6 minutes,
                                                fi = 0.4, C10 = 0.78, C5 = 0.74, C100 = 0.94.

                                                Q5        = FCIA = 1/360 x 0.74 x 190 x 0.06
                                                          = 0.023cumecs - 0.010cumecs roofwater = 0.013cumecs

                                                Q100      = FCIA = 1/360 x 0.94 x 303 x 0.06
                                                          = 0.047cumecs - 0.010cumecs roofwater = 0.037cumecs


House        Pre-development          Post-development flow         Pre-development flow        Post-development flow
number      flow Q5 in cumecs             Q5 in cumecs                Q100 in cumecs              Q100 in cumecs

  29               0.068                       0.013                        0.137                      0.037

So mostly due to new road catchment and roofwater drainage a significant reduction in flow occurs as a result of
development without any detention. Detention for overall flow increases from subdivision will be dealt with at 14
Eagle Drive basin.

A similar order of flow reduction will apply for lots 23, 25, 27, 29, 31, 33 Tait Street.



                                                                                                                 pg. 10

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Other adjacent properties at 35 and 35A Tait Street.

35 Tait Street: Existing Catchment =            0.388Ha,
                                                165m length of grass/treed area at 3.4%, tc = 17 minutes,
                                                fi = 0, C10 = 0.70, C5 = 0.67, C100 = 0.84.

                                                Q5        = FCIA = 1/360 x 0.67 x 124 x 0.388 = 0.090cumecs

                                                Q100      = FCIA = 1/360 x 0.84 x 200 x 0.388 = 0.181cumecs


35 Tait Street: Developed Catchment =           0.115Ha,
                                                Urban lot area at 3.4%, tc = 10 minutes,
                                                fi = 0.4, C10 = 0.78, C5 = 0.74, C100 = 0.94.

                                                Q5        = FCIA = 1/360 x 0.74 x 156 x 0.115
                                                          = 0.037cumecs - 0.030cumecs roofwater (3 lots)
                                                          = 0.007cumecs

                                                Q100      = FCIA = 1/360 x 0.94 x 251 x 0.115
                                                          = 0.075cumecs - 0.030cumecs roofwater
                                                          = 0.045cumecs


House        Pre-development          Post-development flow         Pre-development flow        Post-development flow
number      flow Q5 in cumecs             Q5 in cumecs                Q100 in cumecs              Q100 in cumecs

  35               0.090                       0.007                        0.181                      0.045

Again mostly due to new road catchment and roofwater drainage a significant reduction in flow occurs as a result
of development without any detention. Detention for overall flow increases from subdivision will be dealt with at
14 Eagle Drive basin.

A similar order of flow reduction will apply for lot 35A Tait Street.

Summary of Drainage Improvements/Non Worsening.

The existing area has a number of drainage issues, and as per Callaghan and Toth previous investigations the
best way to resolve the existing issues and cater for new subdivision is to create an overland flow path through
14 Eagle Drive with detention capacity. This solutions proposed will enable a non-worsening outcome for new
subdivision flow in relation to surrounding properties in accordance with Council Planning Scheme Policy.




                                                                                                                 pg. 11

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6.0     CONCLUSIONS
An integrated water management approach has been used to assess the site’s stormwater requirements and
water management needs. In summary:

       Subdivision will improve current flooding situation for many adjacent properties, and provide an overland
        flow path for this subdivision and surrounding area.
       Subdivision catchment follows Council Master Drainage Catchments
       Proposed Bio-retention basin will treat new subdivision flow to Council requirements.
       Proposed Detention basin will lower new subdivision catchment area flow to pre-development levels.




                                                                                                            pg. 12

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7.0   APPENDIX – Council Master Stormwater Catchments




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